1. Field of the Invention
The invention relates to a separate, continuous, dense barrier layer between an MCrAlY basecoat or overlay and a superalloy turbine component, to prevent depletion of Al from the MCrAlY by interdiffusion into the superalloy and to prevent interdiffusion of elements such as Ti, W, Ta and Hf from the superalloy into the coating.
2. Background Information
Numerous overlay and thermal barrier coatings are well know in the gas turbine engine industry as a means of protecting nickel and cobalt based superalloys components, such as blades and vanes, from the harsh oxidation and hot corrosion environments during engine operation. Coatings can be generally classified as overlay and diffusion coatings, providing solely oxidation and corrosion resistance to the superalloy component, and thermal barrier coatings, providing reduced heat transfer between the hot gas path and the cooled turbine component. Generally, thermal barrier coatings are applied over a basecoat of an overlay coating or a diffusion coating.
One type of thermal barrier coating is described in U.S. Pat. Nos. 4,321,310 and 4,321,311. As described therein, a thermal barrier coating is deposited on to a superalloy component (substrate) by first depositing an MCrAlY metal alloy where M is generally nickel, cobalt, or a combination thereof, oxidizing the MCrAlY alloy surface to form an alumina layer in-situ, and depositing a ceramic thermal barrier layer onto the alumina layer.
Other types of thermal barrier coatings utilize ordered intermetallic compounds as the basecoat where aluminum is deposited from the gas phase (U.S. Pat. No. 3,486,927 or liquid phase (U.S. Pat. No. 5,795,659), and heat treated to form a diffusion aluminide intermetallic (typically nickel aluminide, NiAl, cobalt aluminide, CoAl, or mixed (Ni/Co)Al) layer. A modification to the aluminide coating incorporates platinum plating of the substrate prior to gas phase aluminizing to produce a basecoat layer rich in platinum aluminide (PtAl.sub.2) (U.S. Pat. No. 3,692,554). Numerous other examples and modifications can be found in the literature and U.S. Patents.
The thermal barrier coating system utilizes a ceramic top coat, such as yttria stabilized zirconia, applied over the basecoat. The ceramic top coat is typically applied by either electron beam physical vapor deposition (EB-PVD) or by plasma spray. The surface of the basecoat is optimized to maximize adherence between the basecoat and the specific ceramic top coat used. For EB-PVD, the basecoat is usually polished and preoxidized prior to deposition of a columnar ceramic thermal barrier layer. In contrast, plasma sprayed top coats favor a rough basecoat surface and do not require the in-situ formation of an aluminum oxide layer prior to deposition. Plasma sprayed ceramic thermal barrier coatings rely on porosity and microcracks to accommodate strain during service.
Regardless of the type of thermal barrier coating system employed, service life is dependent on the formation and maintenance of an aluminum oxide passive layer at the interface between basecoat and the thermal barrier coating. The aluminum oxide layer forms in-situ during fabrication and grows during subsequent service to provide an oxygen barrier preventing further degradation. Similarly, on overlay coatings (with no ceramic layer), oxidation resistance is dependent on the formation and maintenance of an aluminum oxide layer on the surface of the overlay coating.
Aluminum is required to form and is consumed from the basecoat in the formation of the passive aluminum oxide scale. Aluminum is also consumed during interdiffusion of aluminum from the basecoat into the substrate. Failure of the basecoat occurs when there is insufficient aluminum remaining in the basecoat to form and maintain a coherent alumina scale. Furthermore, interdiffusion of certain superalloy constituent elements to the passive aluminum oxide scale can accelerate the degradation process.
Taylor et al (U.S. Pat. No. 5,455,199) examines modifying MCrAlY basecoat alloy chemistry by incorporating heavy metals such as tantalum, rhenium, and/or platinum into the basecoat to slow diffusion and loss of aluminum to the substrate. The reduced diffusivity is also likely to slow the movement of aluminum to the aluminum oxide scale necessary for forming and maintaining the passive scale. Similarly, Czech et al (U.S. Pat. No. 5,268,238) incorporated 1% to 20% rhenium into the basecoat chemistry to slow interdiffusion and increase corrosion resistance. Furthermore, since the heavy metals are present throughout the basecoat alloy, it is expected that the resulting coating will be expensive.
An alternative is to apply a diffusion barrier at the interface between the MCrAlY basecoat and the superalloy. For example, an aluminide or platinum layer is mentioned as a layer in contact with the substrate to provide basecoat durability in U.S. Pat. No. 4,321,311 (Strangman). A plurality of chromium based layers, each resistant to high corrosion temperatures and with diffusion barrier layers of titanium nitride or titanium carbide between layers, is taught as a turbine blade coating in U.S. Pat. No. 5,499,905 (Schmitz et al.).
Leverant teaches in U.S. Pat. No. 5,556,713 that atomic rhenium deposits help slow diffusion of aluminum out of the basecoat layer. A submicron, diffusion deposit of rhenium atoms, formed by vacuum condensing vaporized rhenium onto the superalloy substrate while simultaneously bombarding the substrate surface with an energetic beam of inert ions, such as argon is used to obtain sufficient bonding of the barrier layer to the substrate. The atomic rhenium deposit has a maximum thickness of 1000 nm (1 micrometer), and is preferably 0.05 micron to 0.2 micron thick. This process would seem to be costly and slow, and to only apply primarily to block diffusion of Al out of the basecoat. It would also seem to be limited to simple geometries involving ion beam bombardment, and the ion beam could cause strain on the superalloy structure.
What is needed is a single process to prevent not only diffusion of elements, such as Al, into the superalloy substrate, but also to prevent diffusion of Ti, W, Ta and Hf from the superalloy into the basecoat, thereby causing degrading of the passive aluminum oxide scale on the basecoat by use of a diffusion barrier composition that also allows sufficient coating adhesion. The process should be cost effective and allow coating of large turbine components.